A typical phantom (or inline) power communications system includes power-sourcing communications equipment and a set of remotely-powered network devices that connect to the power-sourcing communications equipment though a set of network cables. The power-sourcing communications equipment includes a power supply and transmit/receive circuitry. During operation, the power supply provides power to the remotely-powered network devices through the network cables, and the transmit/receive circuitry concurrently exchanges data with the remotely-powered network devices through the same network cables. Accordingly, the users of the remotely-powered network devices are not burdened with having to separately connect their devices to power sources (e.g., wall outlets).
Several conventional approaches exist for provisioning power to remotely-powered network devices over cables having some amount of resistance. One conventional approach, which is hereinafter referred to as the conventional “over-provisioning approach”, involves the equipment manufacture designing the power-sourcing communications equipment for a worst-case scenario in which the power-sourcing communications equipment connects to a maximum number of remotely-powered network devices through network cables at their maximum specified lengths (e.g., 100 meters in accordance with the IEEE 802.3af standard). Under this approach, the equipment manufacturer provisions particular characteristics of the power-sourcing communications equipment for a maximum power draw (e.g., maximum power supplied to each remote device and maximum power loss over each network cable due to the network cables being at their maximum lengths). For example, the manufacturer makes sure the power supply is large enough, that there are enough circuit board power planes or that the circuit board power planes and power converters are robust enough to carry worst case current, and that the fan assembly is strong enough to provide adequate cooling. Another conventional approach, which is hereinafter referred to as the conventional “statistical methods” approach, involves the equipment manufacturer designing the power-sourcing communications equipment based on probable uses of the equipment in the field. For example, the manufacturer may offer two models of power-sourcing communications equipment, namely, a lower-end model which is designed for lower power demand situations, and a higher-end model which is designed for higher power demand situation, and then rely on the customer to select the best-suited model for a particular installation location. There are also industry standards which attempt to provide guidelines for manufacturing certain types of power-sourcing communications equipment. For example, the IEEE 802.3af standard, or the newer IEEE 802.3-2005 standard, which is also called the “Power over Ethernet” (PoE) standard, defines ways to build Ethernet power-sourcing equipment and powered terminals. In particular, the IEEE 802.3-2005 standard identifies ways to deliver certain electrical features (e.g., 48 volts) of D.C. power over unshielded twisted-pair wiring (e.g., Category 3, 5, 5e or 6 network cables, patch cables, patch-panels, outlets and connecting hardware) to a variety of Ethernet devices or terminals such as IP phones, wireless LAN access points, laptop computers and Web cameras.
In the context of the IEEE 802.3-2005 PoE standard where the power-sourcing communications equipment is called the PSE (Power Sourcing Equipment) and the remote device is called the PD (Powered Device), some PSEs include Time Domain Reflectometry circuitry which determines the integrity of the cables, i.e., the data channels.